This invention relates to an apparatus for performing a controlled operation on a workpiece, wherein a table having a workpiece support surface is moveable along a first linear axis and a work instrument is moveable along a second linear axis orthogonal to its first axis to achieve application of the instrument to a workpiece with a high degree of accuracy.
Apparatus of the type with which this invention is concerned may, for example, be used in a photoplotter to position the photohead relative to the workpiece. In such a device, a the photohead relative to the workpiece. In such a device, a requirement for precise accuracy is important because the user may be generating on the workpiece graphic information both miniaturized and intricate. U.S. Pat. No. 4,589,746, having a common assignee with the present invention, also addresses a concern for precise accuracy in a coordinate positioning environment. As is recognized in this patent, drive systems which effect X and Y coordinate movement must, to achieve accurate positioning, possess negligible play or backlash. A way to reduce backlash in a lead screw type drive system is to use a recirculating ball nut in which its balls are pressure loaded against the thread or threads of its lead screw. However, in a drive using a lead screw and a nut, the mass and inertia of both the nut and the lead screw require that a drive motor of significant power be used to move the parts of the system at reasonable accelerations. While such drives have proven to be very accurate in their performance of coordinate plotting operations, as well as being suited for high velocity movements of the workpiece relative to the work element, these drive systems tend to be relatively expensive in terms of manufacturing costs. For example, the lead screw in this type of system must be machined with particular tolerances in order that the desired accuracy of the coordinate control system may be achieved. This type of machining in the manufacturing process is a costly step and, if eliminated, could substantially reduce the price of the unit as a whole. Also, as previously mentioned, the attachment of the ball nut to the work table generates additional mass which must be compensated for in the selection of the size of the motor used to drive the system. Since the mass of the lead screw and the ball nut must be moved by a drive motor, the appropriate sizing of this motor relative to what is being driven is also reflected in its cost.
Other systems utilize gearing, such as a rack and pinion assembly or intermeshing circular gears, as the means for driving an X and Y coordinate positioning system. The intermeshing of gears introduce, however, an undesireable ratcheting effect to the workpiece. Ratcheting between gears occurs as teeth impact on each other. When a gear drive is employed in the environment of a photoplotter, the effects of ratcheting are realized when the user is generating graphic information of extremely minute proportions. When this type of graphic information is being created, the thickness of the line which is being exposed on the photsensitive workpiece is of such a thin thickness as to be sensitive to the effects of ratcheting. The detrimental effects of ratcheting are manifested in a desired straight line being drawn with a discontinuity or a linear irregularity. Furthermore, the cost of manufacturing the component gear parts in bulk becomes a factor given the close tolerances needed to accomplish the desired accuracy of the system. Also, gear drive systems require lubrication and tend to be noisy while also being subject to wear which reduces the accuracy of the component parts.
Accordingly, it is an object of the present invention to provide a dual axis coordinate plotting system, which executes position commands in both the X and Y coordinate axis at a maximum velocity of, for example, 400 inches per minute while also orienting the coordinate driven members in an extremely accurate and smooth manner without the detrimental effects of gear ratcheting.
Yet, another object of the present invention is to provide a coordinate control system having very high tightness in the joint connections while maintaining a low mass in the members which are subject to high inertial forces, thus giving low mass and low inertia to the system.
It is yet another object of the present invention to eliminate the effects of ratcheting by providing friction drive means to both the X and Y coordinate drive systems and to control these drive means by a closed loop feedback system having sensors which determine movements according to the actual X and Y position of the driven members.
A further object of the present invention is to provide a highly accurate coordinate control system of reduced cost by utilizing low mass, low cost structural components to orient the driven members and a reduced size electric motor to drive the low mass system.
A still further object of the present invention is to provide a drive system that does not utilize gears resulting in a quiet, lubrication free, nominal wear device.